Conventional generators convert mechanical energy into electrical energy. Claw pole generators are generally used in motor vehicles, and can be equipped with electrical excitation. Because claw pole generators generate three-phase current, rectification is necessary for the DC voltage vehicle electrical systems common in motor vehicles. Rectifiers having semiconductor diodes and/or active switching elements can be used for this.
Generators that can also be used to drive a vehicle (i.e., can also be operated in motor mode) are used in the field of hybrid vehicles. The objective here is to assist the combustion engine at low rotation speeds at which it is not yet supplying its full torque (boost mode, turbo lag compensation). In addition, thanks to active electric braking (generator mode), the vehicle's kinetic energy can be fed back into the vehicle's electrical system (regeneration). For generator mode, the power converter can be operated as an active rectifier in which the high-side and low-side switches are switched over as a function of phase voltage. For example, the low-side switches are closed when the phase voltage is negative, and the high-side switches when the phase voltage is higher than the battery voltage. A knowledge of the phase currents is important for accurate control of the active braking torque.
The phase currents can be measured via measuring resistors, called “shunts.” As a rule these shunts are installed not directly in the phases of the drive system but instead in a low-side branch of the power converter. An individual-phase current measurement using low-side shunts is available in the field of control application to electric motors. In this context, however, as indicated in German Patent Application No.s DE 103 27 690 B4 or DE 10 2012 217 116 A1, the current measurement must be synchronized with the control application. This is relatively complex. Also problematic in the context of active rectification is the fact that at no point in time are all the low-side switches closed. With current measurements in the low-side branch, however, currents not equal to zero can also be measured when a switch is open, for example due to measurement noise, sensor offsets, etc. This makes evaluation even more difficult.
It is therefore desirable to describe a method with which all the phase currents can be determined in as simple a manner as possible even in an active rectification mode.